In the manufacture of footwear, several steps in the process require that surfaces be bound together by adhesives. Once the surfaces are adhered they may or may not be stitched depending upon the specific footwear construction. Typically, solvent-based adhesives are used for the adhesive applications. The surfaces that are adhered together are often conditioned such as by washing, roughening, etc. in order to ensure a good adhesive bond.
A component in footwear construction is the sole of the footwear. For certain types of footwear the soles are molded and after molding they are adhesively secured to an upper. The unit soles are usually injection molded from thermoplastic materials, e.g. urethanes, polyvinyl chlorides, block copolymers (Kraton.RTM.), polyolefins, etcetera. Prior to the molding step, a mold release is applied to the surfaces of the mold which will contact the sole during the molding step. The agent transfers, at least in part, to the surface of the molded sole.
Typically the next step in the manufacturing process is the attachment of the sole to the upper. The surface of the molded sole where the adhesives will be applied are cleaned to remove the mold release agent and may be abraded for enhanced bonding. This step is critical in that any mold release agent left on the adhesive margin of the sole will contaminate the bond line and result in failure of the adhesive. Adhesive application machines apply a ribbon of adhesive to the periphery of the sole. The cemented surface of the upper is then put into contacting engagement with the adhesive on the sole and pressure applied to insure a good adhesive bond.
The present invention embodies a sole assembly comprising an outer sole, a film of heat activatable adhesive molded into (absorbed) the sole and on the surface (adsorbed) of the sole and a release film over the adhesive. The sole per se is formed by prior art techniques.
The adhesive is an integral part of the sole. This allows the handling of the molded sole assemblies without displacing either the adhesive film or liner prior to the bonding step.
When the molded sole is used in the next step of the bonding process the release liner is simply removed, the adhesive on the sole is activated, and the upper attached. No washing or further sole preparation is necessary.
The adhesive properties are the same or better than prior art adhesives. The chemical nature of the adhesive is not critical. For the adhesive, the various adhesives are selected such that they possess at least the following characteristic--an adhesive which has an affinity for the surface which is greater than the cohesive force of the adhesive layer such that the adhesive bond will not break but rather the substrate will tear.
Further, for the adhesive to be suitable for its use in bonding the outer sole to the upper in addition to causing substrate tearing bonds when forcefully pulled apart, it preferably must meet or exceed the following performance tests:
1. Reactivate at 115.degree.-125.degree. F. PA1 2. Have enough initial tack or grab to adhere the sole to the upper without slipping, prior to fully pressing. PA1 3. Bond to the upper permanently within 8 seconds under pressure. PA1 4. Pass standard heat and water resistance tests (140.degree. F./95% R.H., and 48 hour water soak). PA1 5. Be impervious to oil and plasticizer migration; and PA1 6. Pass 100,000 flex cycles without opening.
Film adhesives generally useful in the practice of the invention would include compounded thermoplastic resins, such as polyurethanes, polyolefins, or ethylene vinyl acetates. Obviously, the adhesive of choice is based upon the composition of the sole to which the upper is to be attached and the composition of the upper.
The release liner used, such as silicone, polyethylene coated paper, polyethylene or polypropylene, can be used for the applications described herein, as long as the release liner provides a weak bond to the face of the adhesive and should be as thin as possible while still serving that function.
The following sole/adhesive/upper combinations are believed suitable for purposes of the invention: polyurethane, polyvinyl chloride, or block copolymer for the soles/polyurethane for the adhesives/leather, fabric, or synthetic (PVC, urethane, etc.) for the uppers; polyethylene, polypropylene, or EPDM blends for the soles/polyolefins for the adhesives/polyolefin film/leather or fabric, for the uppers; and EVA (ethylene vinyl acetate) for the soles/ethylene vinyl acetates or chlorinated rubber compounds for the adhesives/EVA film/leather, fabric, or non-PVC synthetic for the uppers.